i) Field of the Invention
The present invention relates to a drive control apparatus for a thermal head and more particularly to an apparatus for a formation of a control signal for driving heater elements in the thermal head.
ii) Description of the Related Arts
Conventionally, a thermal head is provided with a heater element group or a plurality of heater elements arranged in a matrix form corresponding to print dots, and the heater elements are selectively driven according to print data in order to carry out a print operation directly onto heat sensitive paper or onto paper via an ink ribbon. In the thermal head of this kind, when the print speed is increased, the next driving operation of the heater elements is started before the temperature of the driven heater elements is lowered, and hence the surface temperature of the thermal head is raised to cause uneven printing. Further, when the heat storage temperature of the head is increased, the thermal stress causes cracks or bubbles in the head which reduces the head life. Accordingly, in the conventional thermal head, a past thermal history of the heater elements is detected, and a current application time to the heater elements is controlled according to the detected result.
In FIG. 1, there is shown a conventional drive control apparatus for a thermal head. In this drive control apparatus, print data are stored in a shift register (S.R.) 1 by using a clock signal. To the shift register 1, a plurality of gate circuits 3a to 3n are connected via a latch circuit 2, and the gate circuits 3a to 3n are coupled to respective heater elements 4. In this case, the print data stored in order in the shift register 1 are temporarily held in the latch circuit 2 and are then output to the gate circuits 3a to 3n. Hence, in the gate circuits 3a to 3n, on the basis of the present print data, the preceding print data and a gate signal, a print control signal to be fed to the heater elements 4 is formed. For example, when the print data are "Hi" (print state) at not only the present time but also the preceding time, a control signal of a shorter pulse is formed rather than in the case where the print data are "Hi" at the present time but "Low" (blank state) at the preceding time. As a result, a suitable current application time considering the past heat generation state is given, and a good printing operation can be carried out. Further, in this case, a history taking into account the state of the heater element two times before can be also considered.
In the above-described conventional drive control apparatus for the thermal head, as shown in FIG. 2, when a zigzag half pattern printing operation is carried out, a blank part is partially encroached by the printed part and a good quality printing state can not be sufficiently obtained.
Accordingly, it is considered that the drive time, the drive voltage and the like of the present heater element to be controlled are controlled with reference to not only the print history information of the present heater element but also to the print information of the adjacent heater elements in order to obtain proper printed dots.
However, particular in case of speed printing, since the history of the adjacent heater elements has an influence on the print quality, it is necessary to consider the print history information of the adjacent heater elements in addition to the print history information of the present heater element and the print information of the adjacent heater elements.
Hence, as shown in FIG. 3, it can be considered to drive a present heater element for printing a dot D.sub.A in consideration of not only the present states of dots D.sub.B and D.sub.C of the adjacent heater elements, and the preceding state of dot D.sub.D and a state two times before of dot D.sub.G of the present heater element but also the preceding state of dots D.sub.E and D.sub.F and the state two times before of dots D.sub.H and D.sub.I of the adjacent heater elements.
However, if these dots are all considered, the number of dot bits to be considered is 8, and, when a level separation is executed in consideration of all these particulars, there are 256 level separations. Thus, the circuit structure becomes very complicated, and further it is actually impossible to carry out a rapid transmission of 256 level data to a driver.